46
Please contact DVC regarding the removal of the CCD glass faceplate for UV response down to
the 200nm range ! Do NOT attempt to remove the faceplate, this WILL void the warranty on the
CCD sensor.
8.4.4 What is meant by the dynamic range of a CCD ?
The dynamic range of a CCD (or of any measuring system) is a measure of it's range of useful
operation. For a CCD this relates to light.
Voltage vs. Light curve for the TC-245 CCD
0
50
100
150
200
250
300
350
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Light (lux)
Voltage (mV)
Reaches Vsat at 100lux !
Vsat
Reaches Vsat at 1lux !
Vn = Noise Floor (not to scale)
Vuse = Camera White Clip
Vsat /2
Anti-blooming disabled
Anti-blooming Enabled
Figure 8-3: Exposure Curve showing the Dynamic Range of the Camera
In the case of the DVC camera, the maximum light that can be measured is 0.5 lux. That's
because if more light (than 0.5 lux) is incident on the CCD, the camera will reach 100 IRE (or, in
8 bit digital terms the pixel level of "255").
This operating point is indicated on the above graph as Vuse - the CCD output voltage which will
result in 100 IRE units at the output. This is a camera design parameter; Vuse can be moved up
the saturation curve by decreasing the camera gain and can be moved down the saturation
curve by increasing the camera gain.
Vn (the noise floor, typically 0.08 to 0.12 mV) is a fixed parameter (for a given temperature). The
ratio of Vuse / Vn (and therefore the dynamic range) is completely dependent on our choice of
Vuse; what this means is that we can "trade off" the camera gain (and hence sensitivity) for the
camera dynamic range (hence signal-to-noise ratio).
The other parameter that is important in defining the dynamic range of a camera is the smallest
signal that can be accurately measured by it. The physical parameter that limits the smallest
discernible signal is the dark current noise.
In a TC-245 CCD sensor, the dark current noise (at 25°C) is represented by the 30 electrons that
accumulate at a charge site (often called charge "well") in a normal 1/60 sec integration period if
no light is incident on it (this is the worst case, a more typical number is 20 electrons). These
electrons are often referred to as "noise equivalent electrons" and constitute a noise floor.
Summary of Contents for DigitEyes Series
Page 14: ...9 Figure 4 3 TC 245 Gate Level Drawing Texas Instruments 1994 ...
Page 61: ...56 12 Appendix D Camera Mechanical Drawings Figure 12 1 Camera Mechanical Drawings ...
Page 63: ...58 Figure 13 2 Camera Noise Spectrum Min Gain Bandwidth 10kHz to 4 2MHz ...
Page 64: ...59 Figure 13 3 Camera Noise Spectrum Max Gain Bandwidth 100kHz to 4 2Mhz ...
Page 65: ...60 Figure 13 4 Camera Noise Spectrum Max Gain Bandwidth 10kHz to full ...